1. Technical Field
The present invention relates to a polarization-independent optical isolator, and more particularly, relates to a polarization-independent optical isolator that allows for the isolation of both TM-mode and TE-mode, as well as for the completely preventing any propagation to the backward propagating light in a designed wavelength.
2. Prior Art
An optical isolator is an element which allows a light to transmit only in one direction but prevents the light from propagating in a direction opposite thereto. For example, by arranging the optical isolator at an emitting end of a semiconductor laser, the light output from the laser transmits through the optical isolator, and it is possible to be used the light as a light source for optical fiber communications. Conversely, the light which is going to be input on the semiconductor laser through the optical isolator is prevented by the optical isolator, so that the light cannot be input on the semiconductor laser. Unless the optical isolator is placed at the emitting end of the semiconductor laser, a reflected return light will be incident on the semiconductor laser, and thereby a degradation of oscillation characteristics of the semiconductor laser is caused. Namely, the optical isolator serves to block the light which is going to be incident on the semiconductor laser, and to maintain a stable oscillation without degrading the characteristics of the semiconductor laser.
In optical active elements such as not only the aforementioned semiconductor laser but also an optical amplifier or the like, by unintentional incidence of light from an opposite direction, operating characteristics of the optical active elements may be degraded or unintentional operation may be performed. However, since the optical isolator allows the light to transmit only in one direction, it is possible to prevent the unintentional backward light from being incidence of to the optical active element.
Conventionally, an interference-type optical isolator (waveguide-type optical isolator) 101 shown in FIG. 1 with the perspective view has been proposed as an optical isolator suitable for integration with a semiconductor laser.
The above conventional optical isolator 101 is constituted that a waveguide layer 103 using a semiconductor material is provided on a compound semiconductor substrate 102, a waveguide 104 is formed in the waveguide layer 103, a tapered branching/coupling device 105 is provided on the waveguide layer 103, and further a non-reciprocal phase shifter 106 is provided on the waveguide layer 103. Incidentally, the non-reciprocal phase shifter 106 is provided with a cladding layer 107 composed of a magneto-optical material, and magnetic field applying means 108 for completing magnetization of the magneto-optical material in a predetermined direction, and the non-reciprocal phase shifter 106 is formed that the magnetic field applying means 108 is provided on the cladding layer 107.
FIGS. 2A-2C show an operating principle of the waveguide-type optical isolator 101. The optical isolator 101 (hereinafter, referred to as “waveguide-type optical isolator 101”) is set so that light waves which propagate in two optical waveguides may be in same phase to propagating waves of a forward direction (forward propagating waves) and may be in opposite phases to propagating waves of backward direction (backward propagating waves) which propagate in an opposite direction, by utilizing a phase variation (hereinafter, referred to as “non-reciprocal phase shifting effect”) whose magnitude is different depending on a propagation direction generated in two optical waveguides constituting an optical interferometer (refer to FIG. 2A).
In a case where two light waves are in the same phases, the light waves which input from a central input end 110 is output from a central output end 111 in a tapered branching/coupling device 105 provided on the output side (on the right-hand side in FIG. 1) based on a symmetry of the structure (refer to FIG. 2B).
Meanwhile, in a case where two light waves are in opposite phases, from the symmetry of the structure, since an antisymmetric distribution is formed in the tapered branching/coupling device 105 provided on the input side (in the left-hand side in FIG. 1), the light waves input from a central output end (reflected return light) are not output from the central output end (input end) 110 of the tapered branching/coupling device 105 but are output from undesired light output ends 112 provided on both sides of the central output end 110 (refer to FIG. 2C).
That is, the light wave input from the central input end 110 of the tapered branching/coupling device 105 on the left-hand side in FIG. 1 is output from the central output end 111 of the tapered branching/coupling device 105 on the right-hand side in FIG. 1. Conversely, the light wave input from the central output end 111 of the tapered branching/coupling device 105 on the right-hand side in FIG. 1 is output from the waveguides on the left-hand side in FIG. 1 without entering to the central input end 110 of the tapered branching/coupling 105 on the left-hand side in FIG. 1. As described above, by setting so that two light waves are in opposite phases to a propagating light of the backward direction (hereinafter, referred to as “backward propagating light”), it is possible to isolate the backward propagating light input from the central output end of the branching/coupling device on the right-hand side in FIG. 1.
In order to achieve an operation of branching and coupling characteristics of such a light of the optical isolator, a constant amount of the non-reciprocal phase shifting effect is required. First of all, one of the interfering optical paths is made longer than the other one, and thereby a phase difference independent of the propagating direction between the two optical paths (reciprocal phase difference) is generated. The non-reciprocal phase shifting effect can be generated by arranging a magneto-optical material (material having a magneto-optical effect) in a planar waveguide, externally applying a magnetic field in a direction perpendicular to a propagating direction (transverse direction) in a waveguide plane, and orienting magnetization of the magneto-optical material. Since the non-reciprocal phase shifting effect due to the magneto-optical effect is determined by the relationship between the propagating direction of the light and an orientation direction of the magnetization, the non-reciprocal phase shifting effect is different in a case where the propagating direction is reversed while keeping a magnetizing direction.
Since the magnetic fields are applied in an antiparallel to each other to two waveguides constituting the interferometer in the waveguide-type optical isolator shown in FIGS. 2A-2C, a phase difference of the light waves when each light waves propagate the same distance in two waveguides corresponds to an amount of non-reciprocal phase shifting (difference of the phase variation between the forward propagating wave and the backward propagating wave). Additionally, if a phase difference “+φ” occurs between two waveguides due to the non-reciprocal phase shifting effect to the forward propagating wave, a phase difference “−φ” which is an opposite sign to that will occur to the backward propagating wave.
In addition to the non-reciprocal phase shifting effect caused by the magnetic field, an optical path length difference corresponding to ¼—wavelength is provided in two waveguides constituting the interferometer. It is intended that the light which propagates through a waveguide with longer optical path gives a phase variation which is larger only “π/2” regardless of the direction. Namely, if the waveguide with longer optical path is made to generate a phase difference due to the non-reciprocal phase shifting effect (hereinafter, referred to as “non-reciprocal phase shifting phase difference”) of “−π/2” as compared with a waveguide with shorter optical path to the forward propagating wave, the light waves which propagate through two waveguides are in the same phase to the forward propagating wave. At this time, since the sign of the non-reciprocal phase shifting phase difference is reversed when the propagation direction is reversed, the non-reciprocal phase shifting phase difference “+π/2” is given to the waveguide with longer optical path. This phase difference and the phase difference “+π/2” due to the optical path length difference are added, so that the light will input into the tapered branching/coupling device in the opposite phase state (phase difference π). From the discussion described above, it can be concluded that the non-reciprocal phase shifting phase difference “π/2” is required in the waveguide-type optical isolator shown in FIGS. 2A-2C.
In such a conventional waveguide-type optical isolator, since the characteristic vary significantly according to the polarization state of the input light, a conventional waveguide-type optical isolator has a polarization dependence that operates as an isolator in an only case where the polarized wave of the input light is single polarized wave of the specific case (TM-mode, vertically-polarized wave). Incidentally, an example of such a waveguide-type optical isolator is disclosed in Japanese Patent No. 3407046. In addition, operational verification examples of a waveguide-type optical isolator in the TM-mode are described in Non-Patent Document 1 and Non-Patent Document 2. Furthermore, Non-Patent Document 3 demonstrates by the experiments that the non-reciprocal phase shifting effect is not generated in the TE-mode by measuring the amount of non-reciprocal phase shift generated to the TM-mode.
However, considering the fact that the optical isolator is used in the optical fiber communications, the polarization state of the light wave transmitted through a fiber changes randomly. Therefore, in an optical isolator having the polarization dependence, the utilizable range is limited. In particular, the preventing characteristics (isolation characteristics) of the backward propagating light are important characteristics in use, and for the above reason, it is required that sufficiently large preventing characteristics is obtained.
In this connection, a polarization-independent waveguide-type optical isolator, in which the magnetization structure of the magnetic garnet is schemed, is proposed in Non-Patent Document 4. However, the optical isolator has a configuration that the control of the magnetization structure is complicated and difficult.
With regard to a bulk-type optical isolator, a polarization-independent bulk-type optical isolator having a polarization diversity (polarization separation and composition) configuration is proposed in Non-Patent Document 5.
A waveguide-type optical isolator utilizing the polarization diversity can also be considered, but it is necessary to separate the input light into the TE-mode (horizontally-polarized Wave) and the TM-mode, and thereafter to rotate polarization plane 90 degrees to the TE-mode and pass through an optical isolator, and to output by further rotating a polarization plane and put it back in place. In this case, two waveguide-type optical isolators are required, and thereby it is caused the seriously reversing consequence to the current requirement specifications which downsize the optical isolator. Therefore, it is scarce of practicality.
Further, Non-Patent Document 6 clarifies a good mode conversion characteristics by prototyping a TE-TM mode converter in an asymmetric-type waveguide structure.
Prior Art Document:
    [Non-Patent Document 1]    H. Yokoi, T. Mizumoto, N. Shinjo, N. Futakuchi, and Y. Nakano, “Demonstration of optical isolator, with a semiconductor guided layer that was obtained by use of a nonreciprocal phase shift,” Applied Optics, vol. 39, No. 33, pp. 6158-6164 (2000).    [Non-Patent Document 2]    H. Yokoi, T. Mizumoto, N. Shinjo, N. Futakuchi, Y. Nakano, “Demonstration of an optical isolator with semiconductor guided layer,” Technical report of IEICE, OPE2000-10, pp. 55-60 (2000).    [Non-Patent Document 3]    T. Mizumoto and Y. Naito, “Non-reciprocal propagation characteristics of YIG thin film,” IEEE Trans. on Microwave Theory and Techniques, vol. MTT-30, No. 6, pp. 922-925 (1982).    [Non-Patent Document 4]    O. Zhuromskyy, H. Deoetsch, M. Lohmeyer, L. Wilkens, and P. Hertel, “Magneto-optics waveguide with polarization-independent non-reciprocal phase shift,” IEEE J. Lightwave Technology, vol. 19, No. 2, pp. 214-221 (2001).    [Non-Patent Document 5]    K. Shiraishi, “New configuration of polarization-independent isolator using a polarization-dependent one,” Electronics Letters, vol. 27, No. 4, pp. 302-303 (1991).    [Non-Patent Document 6]    J. Z. Huang, R. Scarmozzino, G. Nagy, M. J. Steel, and R. M. Osgood, Jr., “Realization of a compact and single-mode optical passive polarization converter,” IEEE Photonics Technology Letters, vol. 12, No. 3, pp. 317-319 (2000).